It is known to employ an extremely long string of guns connected together by a connecting tubing in order to simultaneously perforate two or more vertically spaced apart formations located downhole in a borehole. In the past, in order to cause an upper charge carrier to detonate an underlying charge carrier, it has been necessary to utilize the force of the explosion of the shaped charges in the upper charge carrier for detonating a length of prima cord which extends from the interior of one gun to the interior of another.
The use of a length of prima cord in order to connect together guns which are spaced more than 100 feet apart, for example, is cumbersome and unreliable because the guns must be physically tied together by a tubing string as they are lowered into the borehole. This is a difficult task as well as being time consuming and exceedingly dangerous. Under these trying conditions, it is easy to appreciate that the technician occasionally blunders and fails to properly assemble a gun component, whereupon one of the guns fails to perforate the casing. This gun failure necessitates the employment of a costly and difficult remedial action.
Furthermore, when prima cord is used it is often difficult to determine if both guns have been fired. If, for example, the operator relies on the detection of sounds by geophones to determine when the guns have fired, the two sets of sounds may arrive at the geophones so close together that it is impossible to determine if the sound was the result of firing one gun or both. One reason for this problem is that prima cord burns at about 30,000 feet per second, so that if the second gun is 100 feet from the first gun, it will be fired only 1/300 second later.
Another problem in the prior art is that the tubing connecting the two guns is at atmospheric pressure, filled with air, and is subjected to borehole pressures which are sometimes high enough to cause the tubing to collapse.
Another drawback of the use of prima cord to connect the guns is that the entire string of guns, including the connecting tubing, must be hermetically sealed from the well fluids, and since many of the threaded connections are made up in the field, some of these connections occasionally are not made up tightly, causing the connections to leak a sufficient amount to wet and/or foul the gun components, thereby causing the gun to misfire.
It is, therefore, desirable to have made available a reliable jet perforating gun device which is connected to another similar jet perforating gun device by a connecting tubing, wherein the gun string does not depend upon an interconnecting length of prima cord for the lower gun to be detonated in response to the detonation of the upper gun. Preferably, the lower gun should be fired in a manner which delays its firing for a period after the upper gun fires. Moreover, it is desirable to seal the individual charge carriers of the gun string at the shop or laboratory rather than having to depend upon the seals being effected at the job site, thereby reducing the likelihood of leakage of the gun components.
The method and apparatus for simultaneously perforating spaced formations in a borehole in a manner which overcomes the above drawbacks and enjoys some of the desirable features set forth above is the subject of the present invention.